Integrins provide "positional cues" within the context of the extracellular matrix (ECM) regulating cell survival, and proliferation (1). The objective of this proposal is to understand the mechanisms that regulate cellular survival or commitment to apoptosis in response to the integrin ligation state. We will characterize a process, which we define here as IMD (Integrin-Mediated Death), wherein adherent cells die as a result of the expression of unligated integrins. IMD is biologically and biochemically distinct from "Anoikis," a form of apoptosis characterized in vitro in which death is caused by complete loss of adhesion, per se (2). As shown in Preliminary Results, IMD occurs among adherent cells where unligated integrin beta subunits are able to recruit active caspase 8 to a membrane-associated protein complex. Importantly, IMD occurs in a manner independent of FADD, an adaptor protein that recruits caspase 8 during other forms of apoptosis. We hypothesis that IMD represents a novel, yet physiologically relevant, process ensuring that cells associated with an "inappropriate" ECM undergo apoptosis. Integrin antagonists are currently being used in clinical trials to disrupt tumor-associated angiogenesis, yet, it is unclear how these antagonists function at the biological and molecular level. Therefore, the specific aims of the current proposal are designed to expand our understanding of the mechanisms underlying IMD, and to determine the physiological contributions of this process to angiogenesis and tumor growth. The initial aim of this proposal will focus on determining the minimal structural requirements for formation of the complex involving the integrin beta subunit and caspase 8 during IMD. Efforts will be made to determine whether caspase 8 interacts directly or indirectly with integrin. In the second aim, we will evaluate the role of p53 in IMD based on our previous studies (9) and preliminary data implicating p53 as a central player of IMD in angiogenic endothelial cells. In the final aim we will evaluate the role of IMD in vivo, and determine whether angiogenesis and tumor growth can be influenced by the delivery of genes or compounds that promote IMD. These investigations will yield an improved understanding and provide insight into the molecular mechanism(s) by which unligated integrins induce apoptosis in vitro, and in vivo.